We have developed techniques whereby single auditory-nerve fibers can be filled with horseradish peroxidase (HRP) after their response properties have been determined by recording discharge patterns intracellularly. After histological processing, each labelled fiber can be traced to its endings inthe cochlea where detailed observations can be made of the nature of the neuron, its synaptic connections, and the condition of the sensory cell(s) that are innervated by that fiber. Because the HRP reaction rpoduct is electron dense, the morphological analysis can be performed at both the light and electron-microscopic levels. The studies proposed here should provide us with definitive data on some basic questions concerning the relation between structure and function in the peripheral auditory system. A definitive map relating cochlear locus and characteristic frequency will be established for the cat. It should also be possible to determine the anatomical basis for the functional classification of auditory nerve fibers according to threshold and spontaneous discharge rate. The new labelling techniques will also be applied to the study of noise induced cochlear pathology. Specific physiological abnormalities at the single unit level can then be associated with the morphological condition of the relevant hair cells and nerve fibers. We propose to study the pathological changes associated with a wide range of functional losses, from the slight to the profound, and to investigate these correlations at a number of post-exposure survival times, from the chronic to the acute. From these studies we expect to increase greatly our understanding of the cellular bases for sensorineural hearing disorders.